1. Field of the Invention
The present invention relates to scale inhibitors comprising an organic synthetic high polymer as their active component.
More particularly, the invention relates to a chemical for inhibiting or preventing formation of scale which occurs in aqueous systems owing to increases in the concentrations of hardness components, heavy metal ions, anions, etc., increased alkalinity, increased temperature and the like.
2. Description of the Prior Art
Typical of aqueous systems wherein scale poses problems are boiler water, slag cooling water, industrial cooling water used by a single passage or recycled, water from the coagulating sedimentation treatment of water having collected therein the dust from the exhaust gases of blast furnaces or converters, etc.
Typically, scales include calcium carbonate, calcium phosphate or like calcium-type scale, and zinc phosphate, zinc carbonate, zinc sulfate, zinc hydroxide or like zinc-type scale.
Problems involved in the formation of such scale will be described below in detail.
Various effective chemicals have heretofore been developed and used for inhibiting calcium carbonate scale in industrial water systems. These chemicals include, for example, phosphorus-containing compounds (polyphosphoric acid salts and phosphonic acids), polyacrylic acids, copolymers of acrylic acid and methyl or ethyl ester of (meth)acrylic acid (see Unexamined Japanese Patent Publication No. SHO 53-2393), copolymers containing a 2-hydroxyalkyl (meth)acrylate (see Examined Japanese Patent Publication No. SHO 54-29315), etc.
Among these chemicals, phosphorus-containing compounds, especially inexpensive polyphosphoric acids, are widely used. Also widely used for inhibiting the corrosion of devices or apparatus are systems incorporating such a phosphorus-containing compound singly or in combination with a zinc ion donating compound.
The presence of the phosphorus-containing compound of course inhibits, or prevents the formation of, calcium carbonate scale in the system, whereas the system encounters another problem in that the phosphorus-containing compound itself is liable to form calcium phosphate scale or zinc phosphate scale. Thus, the use of the compound is invariably limited. In recent years, furthermore, it is generally desired to use industrial water as recycled if possible, instead of a single passage, for savings in water, so that the substances in the water are retained therein for a prolonged period of time and become concentrated. The phosphorus-containing compound, when present in such water, hydrolyzes to release phosphate ions, which in turn react with hardness components in the water or with zinc ions added thereto to inevitably form calcium phosphate scale or zinc phosphate scale.
Accordingly, it may be attempted to use a non-phosphorus type scale inhibitor conjointly with the phosphorus compound, but the inhibitor, even if effective on calcium carbonate scale like those mentioned above, actually exhibits little or no effect on calcium phosphate scale or zinc phosphate scale when used in a usual amount.
It also appears likely to use in combination with the phosphorus compound acrylic acid-methacrylic acid-itaconic acid copolymer which is known as a precipitation inhibitor for calcium phosphate (see Unexamined Japanese Patent Publication No. SHO 60-6777) and which is relatively similar in structure to the scale inhibitor of the present invention. However, when used in a small amount, the copolymer is not fully effective, actually failing to inhibit phosphate scale.
On the other hand, the dust collecting water used for blast furnace exhaust gas contains iron oxide and iron hydroxide, which are the main components of the dust, and further zinc ion, calcium ion, magnesium ion, etc. in large quantities and is not reusable as it is by recycling. Accordingly, a recycling system is widely used in which the dust collecting water is subjected to coagulating sedimentation treatment for the removal of the dust components before reuse. The coagulating sedimentation treatment is conducted generally by adding an alkali (e.g. caustic soda) to the dust collecting water to render the water alkaline and also adding a high polymer coagulant (such as polyacrylamide). When the treatment is conducted, for example, in a thickener, iron oxide, iron hydroxide, zinc ion, etc. can be removed efficiently in the form of a sediment by being coagulated and made sparingly soluble.
Nevertheless, the treatment results in another problem in that the system, which is made alkaline, is liable to permit formation of calcium carbonate scale especially on the surface of piping, although the dust component and zinc ion are removable. It therefore appears useful to lower the alkalinity to the greatest possible extent, but if the pH is reduced to less than 7.8, the zinc ion precipitation removal efficiency seriously decreases to entail the problem that zinc-type scale occurs although calcium carbonate scale is inhibited.
Consequently, the conventional coagulating sedimentation treatment requires the double chemical process of giving the dust collecting water an alkalinity of at least 7.8 in pH and further adding a calcium carbonate scale inhibitor to the resulting water (see Unexamined Japanese Patent Publication No. SHO 53-2393, Examined Japanese Patent Publication No. SHO 54-29315, etc.). Thus, the prior-art treatment is disadvantageous with respect to procedural efficiency and economy.
To overcome the drawback, investigations have been made as to the use of scale inhibitors after a neutral coagulating sedimentation treatment, whereas the above-mentioned known calcium carbonate scale inhibitors are ineffective for the inhibition of zinc-type scale when used in a usual amount and are difficult to use in neutral treatment.
The foregoing problems encountered with various aqueous systems would be solved by chemicals which are capable of inhibiting both calcium phosphate scale and zinc-type scale.
In this regard, we previously accomplished an invention providing a method of inhibiting calcium phosphate scale with use of a specific acrylic acid-methacrylic acid copolymer (about 1:4 to about 4:1 in copolymerization ratio and about 500 to about 15,000 in molecular weight) as disclosed in Examined Japanese Patent Publication No. SHO 59-5038.
However, the copolymer was found insufficient in its effect to inhibit calcium phosphate scale and also zinc-type scale.
Unexpectedly, nevertheless, we have found that the effect to inhibit calcium phosphate scale and zinc-type scale can be remarkably improved by introducing a C.sub.1-8 alkyl ester residue of acrylic acid or methacrylic acid partially into the acrylic acid-methacrylic acid copolymer.